A. Field of the Invention
This invention relates to the field of key telephone systems and in particular to providing music to lines of such systems which are put on hold.
B. Background Art
A typical key telephone system may have 12 key stations each station being a telephone of a conventional type, 6 central office lines each capable of handling either an incoming call, an outgoing call, or an intercom call, a Key Service Unit (KSU) for connecting key stations and central office lines as required by controlling a switching matrix, and trunk lines connecting the central office lines of the office telephone system to the external communication network.
The KSU in these systems typically include a microprocessor which controls the overall operation of the system including the switching matrix and various relays within the system. The switching matrix, under the control of the KSU, must have the capability of connecting each of the key stations to each of the central office lines in order to permit all key stations to receive incoming calls on any of the central office lines and conversely to allow all key stations to use any of the central office lines to make an outgoing call.
The central office lines are in turn connected to the trunk lines by a central office interface. Within the central office interface there is a muting relay, sometimes called a "C" relay, for each central office line. One purpose of this muting relay is reducing wear on a pulse relay with the KSU by switching in an RC network. Another purpose is to switch a shunt resistance across a transformer within the interface to minimize transformer spikes. An additional purpose is to switch a load resistor termination onto the central office line, and thereby onto the trunk line, when an outside call is put on hold. Hence, the name muting relay.
It is often desirable, when a call is put on hold, to supply music to the central office line on which the call is routed rather than muting the line. Music supplied to the central office line is transmitted through the trunk line to the party who is put on hold. This has been accomplished several different ways.
One way has been to increase the size of the switching matrix. This has required supplying an extra line to carry music in the matrix. This line, like the lines of all the key stations, may be coupled to any of the central office lines by the switching matrix, thereby allowing the music signal to be applied to any of the central office lines which may be in use for an incoming call. Such an enlargement of the matrix, of course, has increased the complexity and cost of the matrix as well as the complexity of the coding scheme which was required for the microprocessor to control the matrix.
Another method has been to supply music to one of the existing key station lines. This method used the already existing architecture and required no expansion of the matrix and no adjustment to, or increase in, the complexity of the codes by which the microprocessor of the KSU controlled the matrix. However, the disadvantage was that the system handled one less key station because the line which was used to carry the music signal was no longer available for connection to a key station.
Other prior alternatives have been additional relays or switches in the interface between the central office lines and the external trunk lines. A line carrying a music signal could thereby be selectively coupled and decoupled to the interface for a given trunk line whenever the call on that trunk line was put on hold. This eliminated the need to expand the matrix or to decrease the capacity of the overall system. However, it greatly increased the cost of the system since an individual relay would have to be provided for each of the separate central office lines. In addition, control lines and control circuitry for each of these additional relays had to be provided as well as additional microprocessor programming to control the relays.
It is, therefore, an object of the invention to provide music on hold without enlarging the size of the switching matrix between key station lines and central office lines or decreasing the number of key stations which the matrix may switch.
An additional object of the invention is to provide music on hold without providing additional switching relays or switching relay control circuitry.